1. Field of the Invention
The present invention relates to optical media for the storage of data, and more particularly to a method for optical writing servo fields on an optical medium by changing an optical quality of the optical medium.
2. Brief Description of Prior Developments
Optical media that store data along tracks have required methods for keeping a tracking head aligned with the center of a track, so that errors in reading and writing data do not occur.
Previously, conventional far-field optical data storage and tracking methods have predominantly used the continuous composite servo (CCS) approach. CCS is an analog method in which continuous tracking data is extracted from information concerning light diffracted from grooves positioned in relation to the tracks. In a continuous feedback fashion, data from the light diffracted by the grooves indicates to a control circuit where the tracking head should move in order to read the track most effectively.
Another previous method is the sampled-servo method. In this method, embossed pits are positioned into the surface of the optical medium in relation to the center of data tracks also for the purpose of providing tracking information. As the read/write head reads data, it also passes over the areas where the embossed pits have been positioned. If a signal is received by a servo controller corresponding to these servo fields, then the servo controller directs the head back to the center of the track. By encoding the servo fields differently than the data along the tracks, e.g., by encoding the servo fields with different frequency information than the data track, the read/write head not only can read data, but also can reveal information to the controller about its position with respect to the servo fields. In this fashion, tracking may be accomplished even in a near-field optical system.
However, recent approaches to high density optical recording employing near-field methods and other methods tending to reduce the spot size of a laser beam have emphasized the need for novel approaches for following a track on an optical medium. As tracks on an optical medium become thinner in the radial direction due to new capabilities and advances, correspondingly, the beneficial result is that the tracks can be placed closer together. Accordingly, the tolerance and precision of a tracking head in tracking must also increase. The CCS method generally can not be implemented because it is inherently a far-field approach requiring both a servo detection path and a data detection path. As spot size is decreased, the illumination on the optical disk may not be wide enough to encompass both information from the data track and information from adjacent grooves. Accordingly, the CCS method may not be applicable to these near-field approaches. In any event, the dual data path increases both manufacturing time and cost.
Thus, for some near-field approaches, the sampled-servo method has advantages because these approaches utilize the tracking head as both the writing and detection mechanism. Without a mechanism to monitor the diffraction of light from grooves, the sampled-servo method has been the only alternative. However, the sampled-servo method is not without disadvantages. The addition of embossed pits can decrease the overall data storage capacity of the optical medium because an embossed pit has inherent thickness. Also, in a stamping/replicating process, essentially a master stamping disk is produced bearing a template of the optical servo pattern. This master disk is then pressed against the optical disk under a pressure of several tons per square inch. The significant amount of pressure transfers the servo track pattern from the master disk to the medium. The stamping, or replication processes, however, have an associated yield due to such phenomena as stamper wear, injection mold deformation or other imperfect replication steps. To ensure quality, the sampled-servo method requires additional manufacturing steps, thus increasing the time and cost of producing the optical medium.
Furthermore, embossing pits into a sensitive optical medium creates cavities into the surface of the optical medium. This allows for particles, such as dust and other debris particles, to accumulate and become embedded into the pits which can degrade a tracking signal, and decrease the overall accuracy of reading or writing data.
In addition, if a disk is not hubbed before it is servo written, there is increased probabilility that errors in 1f runout will occur due to hubbing errors. These hubbing errors require adaptive algorithms using digital signal processing hardware which creates additional design and cost expense for a disk drive. On the other hand, by providing a system where the disk can be hubbed first, and then optically servo written, the disk tracks and corresponding optical servo fields are concentric with the hub or center of the optical medium.
Moreover, the ability to produce servo fields that correspond exactly to the optical property exploited for signal detection is advantageous. By example, magneto-optical drives detect data by sensing the rotation of polarization due to the Kerr effect. Detection of servo fields is accomplished by detecting the difference in reflectivity from the disk due to the destructive interference caused by a pit written to be one-quarter of a wavelength in depth. The drive therefore must switch between two detection modes when transitioning from data to sector areas.
In contrast, for phase-change recording, a short (less than 100 ns) burst of laser light converts a tiny spot on the medium""s highly reflective crystalline surface to the less reflective amorphous, or semicrystalline state, the conversion occurring upon rapidly heating the material to a temperature above its melting point, then rapidly quenching it, xe2x80x9cfreezingxe2x80x9d it into the amorphous state. Restoring the storage medium to its original state is done by heating the bit locations to a temperature below the material""s melting point, but for an xe2x80x9cextendedxe2x80x9d period of time (on the order of 10xe2x88x925s).
However, a spot on a medium""s crystalline surface can also be differently, xe2x80x9cpermanentlyxe2x80x9d or irreversibly written. For example, xe2x80x9cLaser-Induced Multiple Phase Transitions in Ge-Te Films Traced by Time-Resolved TEM,xe2x80x9d by O. Bostanjoglo and P. Thomsen-Schmidt, Applied Surface Science, Elsevier Science Publishers, pp. 136-141 (1989), illustrates that several different phase structures may be generated for at least one phase changed media composition. In a relevant portion, the article states that complex multiple phase transitions were found by time resolved (TEM) to proceed in laser pulse-annealed Ge-rich GeTE films.
Additionally, xe2x80x9cProgress of Erasable Phase-Change Materials,xe2x80x9d by M. Chen and K. A. Rubi, S.P.I.E. Vol. 1078xe2x80x94Optical Data Storage Topical Meeting, pp. 150-156 (1989), discusses both a metastable and stable crystalline phase for certain optical storage media. The article states that the activation energy barrier between the metastable and the stable crystals is usually very high, and data stability at ambient temperatures is not expected to be a problem. For further background, Progress of Phase-Change Single-Beam Overwrite Technology,xe2x80x9d by Trao et al., S.P.I.E. Vol. 1078xe2x80x94Optical Data Storage Topical Meeting, pp. 2-10 (1989), explains the process and characteristics of graying of the media, such that the gray portions can not be reversibly changed back to the crystalline state. The article discusses that the diffusion length of atoms in optical media may be altered (longer diffusion lengths) for the amorphous state so that the transition to the crystalline state correspondingly becomes longer. This diffusion length alteration may be effected, for example, with multiple overwrites and/or increased amorphization heating periods.
These techniques, however, have not been applied to create optical servo marks on an optical medium; and as the above background description discusses, there is still a need in the art for a mechanism or method for optically servo writing an optical medium. As the detailed description of preferred embodiments will illustrate, the present invention provides a method for optical servo writing and/or tracking servo fields for savings in efficiency, cost and/or time over the servo writing techniques of the prior art.
The present invention provides optical methods for servo writing and reading servo fields in optical media for savings in efficiency, cost and/or time. The present invention implements techniques for changing the crystalline structure of an optical or magneto-optical medium to create optical servo fields. By transmitting a laser beam onto an optical or magneto-optical medium, the crystalline structure is altered at the portions of the substrate of the medium preselected for servo fields. These techniques create optical servo fields and include near-field and far-field laser techniques, different types of lenses, different types of techniques for providing sub-wavelength apertures on a laser-emitting device, lithographic methods and maskless lithography.
The present invention also relates to an optical medium for the storage of data which has portions of its substrate with a different crystalline structure than the rest of the substrate of the optical medium, so that these portions of the substrate can serve as optically created servo fields for a tracking device.